This article explains why balance holes exist, what problem they solve, and how they contribute to reliable, long-term pump operation in real-world applications.
The holes in a pump impeller are there to reduce axial thrust—a force that pushes the impeller along the shaft and places stress on bearings, seals, and internal components.
Without proper control of this force, pumps suffer from premature wear, higher maintenance costs & reduced service life.
A well-designed centrifugal pump manages several dynamic forces during operation.
Of these, two are critical:
Axial forces act parallel to the centreline of the shaft, pushing the impeller either toward or away from the suction side of the pump.
Radial forces act at right angles to the shaft, pulling the impeller sideways depending on operating conditions and flow.
Both forces exist in every centrifugal pump. Good pump design doesn’t eliminate them—it controls them to prevent damage and premature wear.
Axial forces are created by pressure differences across the impeller.
In simple terms:
One side of the impeller is exposed to higher pressure than the other
That pressure imbalance generates a force along the shaft
The largest contribution comes from pressure acting on the impeller shroud
Depending on the impeller design (open, semi-open, or closed), this force can be significant.
Key takeaway: Axial forces are created by pressure imbalance across the impeller, not by pump size or speed alone.
There are several proven engineering methods used to manage axial thrust:
In large multistage pumps, impellers can be arranged to face opposite directions.
For example:
Three impellers face one way
Three face the opposite way
This balances axial forces across the shaft.
Common in high-pressure multistage pumps, balance drums absorb axial load before it reaches the bearings.
Impellers with two inlet eyes positioned 180° apart balance pressure evenly across the impeller, significantly reducing axial thrust.
In the broader pump market, the majority of end-suction pumps use semi-open or single-shroud impellers. These are popular because they:
Are more cost-effective to manufacture
Perform well across a wide range of applications
However, this design creates a downside:
higher unbalanced axial forces acting toward the suction end of the pump.
In some operating conditions, axial thrust in end-suction pumps can reach hundreds of kilograms of force, placing heavy load on thrust bearings.
One way to handle axial thrust is to install larger thrust bearings.
But this introduces new issues:
Larger bearings require larger shafts and housings
Pump size and cost increase
Maintenance and replacement costs rise
Efficiency and compactness are compromised
This is not always the most practical or economical solution.
For many industrial applications, this approach solves the symptom rather than the cause—and increases total cost of ownership.
Balance holes drilled into the impeller shroud allow high-pressure fluid to equalise across the impeller.
The result:
Axial thrust is significantly reduced
Bearing loads are lowered
Seal life improves
Overall pump reliability increases
Yes, balance holes cause a very small efficiency loss—but it is negligible compared to the gains in durability and service life. In most industrial applications, the trade-off is more than justified.
Key takeaway: Balance holes reduce axial thrust at the source, rather than compensating for it with larger mechanical components.
Every detail of a pump is engineered for a reason—even the smallest ones.
Balance holes in an impeller are a simple but highly effective way to:
Control axial thrust
Protect bearings and seals
Reduce maintenance
Extend pump life
What looks like a minor design feature is actually a critical reliability decision—one that reflects how a pump is engineered for long-term operation, not just initial performance.